As it is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which the tube is formed continuously from the web-fed packaging material. More specifically, the web of packaging material is unwound off a reel and fed through a station for applying a sealing strip of heat-seal plastic material, and through an aseptic chamber on the packaging machine, where it is sterilized, e.g. by applying a sterilizing agent such as hydrogen peroxide, which is subsequently evaporated by heating, and/or by subjecting the packaging material to radiation of appropriate wavelength and intensity.
The web of packaging material is then fed through an aseptic chamber, where it is maintained in a sterile-air environment, and through a number of forming assemblies which interact with the packaging material to fold it gradually from strip form into a tube shape.
More specifically, a first portion of the sealing strip is applied to a first longitudinal edge of the packaging material, on the face of the material eventually forming the inside of the packages; and a second portion of the sealing strip projects from the first longitudinal edge.
The forming assemblies are arranged in succession, and comprise respective roller folding members defining a number of compulsory packaging material passages varying gradually in section from a C shape to a substantially circular shape.
On interacting with the folding members, the second longitudinal edge is laid on the outside of the first longitudinal edge with respect to the axis of the tube being formed. More specifically, the sealing strip is located entirely inside the tube, and the face of the second longitudinal edge facing the axis of the tube is superimposed partly on the second portion of the sealing strip, and partly on the face of the first longitudinal edge located on the opposite side to the first portion of the sealing strip.
Packaging machines of the above type are known in which the first and second longitudinal edge are heat sealed to form a longitudinal seal along the tube, which is then filled with the sterilized or pasteurized food product, and is sealed and cut along equally spaced cross sections to form pillow packs, which are then folded mechanically to form respective parallelepiped-shaped packages.
More specifically, the heat-seal operation comprises a first heating step to heat the second longitudinal edge without the sealing strip; and a second pressure step to compress the sealing strip and the longitudinal edges.
The first heating step melts the polyethylene layer of the second longitudinal edge, which transmits heat by conduction to the first longitudinal edge and the sealing strip, so as to melt the polyethylene layer of the first longitudinal edge and the heat-seal material of the sealing strip.
At the second pressure step, the tube is fed between a number of first rollers outside the tube, and at least one second roller inside the tube.
More specifically, the first rollers define a compulsory circular passage for the tube of packaging material, and have respective axes in a plane perpendicular to the path of the tube.
The second roller has an axis parallel to the axis of a corresponding first roller, and presses the sealing strip and the longitudinal edges of the packaging material against the corresponding first roller, so that the heat-seal material of the sealing strip and the polyethylene layers of the longitudinal edges blend completely and form the molecular bonds defining the longitudinal seal of the tube.
More specifically, the second roller cooperates with a surface of uneven thickness, that is, defining a step along the centreline of the sealing strip. The second portion of the sealing strip, in fact, is superimposed directly on the inner face of the second longitudinal edge, whereas the first longitudinal edge is interposed between the inner face of the second longitudinal edge and the first portion of the sealing strip, so the first portion is closer to the axis of the pressure roller than the second portion.
Because of this difference in thickness, highly uneven pressure is exerted on the sealing strip. More specifically, pressure is maximum at the step, and is greater on the first portion than the second portion of the sealing strip.
The danger therefore exists of the pressure on the second portion of the sealing strip not being sufficient to seal the second portion completely to the face of the second longitudinal edge inside the tube, thus resulting in a discontinuous longitudinal seal.
A need is felt within the industry for a packaging machine provided with pressure rollers capable of exerting sufficient pressure on the whole sealing strip to ensure complete blending of the polyethylene, and thereby prevent the formation of a discontinuous longitudinal seal.
It is an object of the present invention to provide a packaging machine designed to achieve the above in a straightforward, low-cost manner.